Equipment or machines used in processing wafers to fabricate integrated circuits are typically automated to reduce exposure of personnel to chemicals, speed production and reduce costs. The machines often have several stations to perform various processes on the wafer and include automated wafer handling devices to move wafers between stations in a machine. Accordingly, each station and wafer handling device includes some sort of wafer holder. It is important that each station and wafer handling device be able to detect or sense whether a wafer is present in its wafer holder. Fast and accurate wafer sensing is needed to optimize the performance of the machine.
Conventional wafer sensors typically use optical sensing methods. For example, a light beam may be directed so as to impinge on a wafer present in the wafer holder. In a through beam system, a wafer present in the wafer holder would block transmission of the light beam to a light sensor. The light sensor would then indicate a wafer is present if the light beam is blocked, and would indicate the absence of a wafer if the light beam is detected. In a reflective beam system, presence of a wafer in the wafer holder would reflect a transmitted light beam to a light sensor. Thus, in this type of sensor, the light sensor would indicate the presence of the wafer if the reflected beam is detected, and would indicate the absence of a wafer if the reflected beam is not detected.
However, these optical systems are not effective in a wet environment due to refraction of the light beam by fluid in the wet environment. For example, in some types of processing machines, the wafers are kept wet when being transferred to one or more stations. Typically, in these types of systems, the processing machines wet the wafers with deionized (DI) water to keep slurry or other chemicals from drying and staining the surface prior to the controlled drying step. It can also help reduce scratching when the wafer is transferred to the wafer holder. As a result, a thin film of the DI water is formed on the wafer upon which the wafer rests in the wafer holder. This film of DI water helps reduce the risk of the wafer holder scratching the wafer. However, the DI water can cause refraction of the light beams used by optical wafer sensors, which tends to cause inaccurate sensing by the optical wafer sensors.
Another type of conventional wafer sensor uses a vacuum technique to sense the presence of a wafer. In this type of wafer sensor, holes are formed in the wafer holder through which a vacuum is drawn. A wafer present in the wafer holder would block the holes, allowing a vacuum to be formed in the holes. This vacuum would then be detected by a vacuum pressure transducer or switch. However, because air is compressible, the time required to detect the vacuum could take several seconds. This relatively long sensing time tends to undesirably reduce machine through-put. In addition, wafer warpage or particles on the wafers may prevent complete blockage of the holes, thereby preventing the vacuum from forming and causing inaccurate wafer sensing. Liquids may also enter the vacuum system and decrease reliability of wafer retention. Accordingly, there is a need for a fast, accurate wafer sensor for use in a wet environment.